Nickel-based superalloy having high resistance to hot-corrosion for monocrystalline blades of industrial turbines

ABSTRACT

Nickel-based superalloy, suitable for monocrystalline solidification, having the following composition by weight:  
                                           Co:    4.75 to 5.25%         Cr:    11.5 to 12.5%         Mo:    0.8 to 1.2%         W:    3.75 to 4.25%         Al:    3.75 to 4.25%         Ti:      4 to 4.8%         Ta:    1.75 to 2.25%         C:   0.006 to 0.04%         B:   ≦0.01%         Zr:   ≦0.01%         Hf:   ≦1%         Nb:   ≦1%                   Ni and any impurities: complement to 100%.

TECHNICAL FIELD

[0001] The invention relates to a nickel-based superalloy which isadapted to the manufacture of fixed and movable monocrystalline bladesof industrial gas turbines by directional solidification.

BACKGROUND OF THE INVENTION

[0002] Nickel-based superalloys are the most high-performance materialsused today in the manufacture of movable and fixed blades of industrialgas turbines. The two principal features required until now of thesealloys for those specific applications have been good resistance tocreep at temperatures of up to 850° C. and very good resistance tohot-corrosion. Some reference alloys currently used in this field aredesignated IN738, IN939 and IN792.

[0003] Blades manufactured using those reference alloys are produced byconventional casting using the lost-wax process and have apolycrystalline structure, that is to say, they are constituted by thejuxtaposition of crystals which are orientated in a random mannerrelative to each other and which are called grains. Those grains arethemselves constituted by an austenitic matrix gamma (γ) based onnickel, in which hardening particles of the phase gamma prime (γ′) aredispersed whose base is the intermetallic compound Ni₃Al. This specificstructure of the grains gives those alloys a high level of creepresistance up to temperatures in the order of 850° C., which ensures thelongevity of the blades, for which service lives of from 50,000 to100,000 hours are generally sought. The chemical composition of alloysIN939, IN738 and IN792 has further been determined to give themexcellent resistance to the combustion gas environment, in particular inrespect of hot-corrosion, a phenomenon which is particularly aggressivein the case of industrial gas turbines. Significant additions of chrome,typically of from 12 to 22% by weight, are thus necessary to give thosealloys the necessary resistance to hot-corrosion for the applicationsconcerned. From the point of view of resistance to creep, the order ofthe alloys is: IN939<IN738<IN792. From the point of view of resistanceto hot-corrosion, the order is the reverse, that is: IN792<IN738<IN939.

[0004] In order to improve the performance of industrial gas turbines interms of output and consumption, one method consists in increasing thetemperature of the gases at the turbine inlet. This consequently makesit necessary to be able to provide alloys for turbine blades which cantolerate operating temperatures which are higher and higher, whilstretaining the same mechanical features, in particular in terms of creep,in order to be able to achieve the same service lives.

[0005] The same type of problem has been posed in the past in the caseof gas turbines for turbo-jets and turbo-engines for aeronauticalapplications. In this case, the selected solution consisted in changingfrom blades, known as polycrystalline blades, which are produced byconventional casting to blades, known as monocrystalline blades, that isto say, which are constituted by a single metallurgical grain.

[0006] Those monocrystalline blades are manufactured by directionalsolidification with lost-wax casting. The elimination of grainboundaries, which are preferential locations for creep deformation atelevated temperature, has allowed the performance of nickel-basedsuperalloys to be increased spectacularly. Furthermore, the process ofmonocrystalline solidification allows the preferred orientation ofgrowth of the monocrystalline component to be selected and, in thatmanner, the orientation <001> which is optimum from the point of view ofresistance to creep and thermal fatigue to be chosen, those two types ofmechanical stress being the most disadvantageous for turbine blades.

[0007] However, the chemical superalloy compounds developed formonocrystalline turbine blades for aeronautical applications are notsuitable for blades for terrestrial or marine applications, known asindustrial applications. Those alloys are determined in order to promotetheir mechanical resistance up to temperatures greater than 1100° C.,and this to the detriment of their resistance to hot-corrosion. In thatmanner, the concentration of chrome of the superalloys for aeronauticalmonocrystalline turbine blades is generally less than 8% by weight,which allows volume fractions of the γ′ phase in the order of 70% to beachieved, which levels are advantageous for resistance to creep atelevated temperature.

[0008] A nickel-based superalloy which is rich in chrome and which issuitable for the monocrystalline solidification of components ofindustrial gas turbines is known by the designation SC16 and isdescribed in FR 2 643 085 A. Its concentration of chrome is equivalentto 16% by weight. The features concerning the creep resistance of alloySC16 are such that the alloy provides, relative to the polycrystallinereference alloy IN738, an increase in operating temperature ranging fromapproximately 30° C. (830° C. instead of 800° C.) to approximately 50°C. (950° C. instead of 900° C.). Comparative tests for cyclicalcorrosion at 850° C. in air at atmospheric pressure with Na₂SO₄contamination showed that the resistance to hot-corrosion of alloy SC16was at least equivalent to that of the reference polycrystalline alloyIN738.

[0009] Hot-corrosion tests have been carried out on alloy SC16 by themanufacturers of industrial turbines on their own test benches. In verysevere environments, which are representative of extreme operatingconditions, it has been shown that the resistance to hot-corrosion ofthat alloy remained inferior to that of alloy IN738.

[0010] Furthermore, the increasing demand from those manufacturers foran increase in the operating temperature of gas turbines gives rise tothe need for superalloys for blades to have a resistance to creep whichis increased still further.

SUMMARY OF THE INVENTION

[0011] The-problem-addressed by the invention is to provide anickel-based superalloy having a resistance to hot-corrosion in theaggressive combustion gas environment of industrial gas turbines whichis at least equivalent to that of reference polycrystalline superalloyIN792, and having a resistance to creep which is greater than or equalto that of reference alloy IN792 within a temperature range of up to1000° C.

[0012] This superalloy must in particular be suitable for manufacture offixed and movable monocrystalline blades having large dimensions (up toseveral tens of centimeters in height) of industrial gas turbines bydirectional solidification.

[0013] Furthermore, this superalloy must demonstrate goodmicrostructural stability in respect of the precipitation of fragileintermetallic phases which are rich in chrome when maintained forsustained periods at elevated temperature.

[0014] More specifically, an alloy compound is sought which ensures:

[0015] optimised resistance to hot-corrosion, in any case at leastequivalent to that of reference polycrystalline super-alloy IN792, andthis in various environments which are representative of that forcombustion gases of industrial turbines;

[0016] a maximum volume fraction of hardening precipitates of the γ′phase in order to promote resistance to creep at elevated temperature;

[0017] resistance to creep up to 1000° C. which is superior to that ofreference polycrystalline alloy IN792;

[0018] a tendency to homogeneity by completely placing in solutionparticles of the γ′ phase, including the γ/γ′ eutectic phases;

[0019] the absence of precipitation of fragile intermetallic phaseswhich are rich in chrome, starting from the a matrix, when maintainedfor sustained periods at elevated temperature;

[0020] a density which is less than 8.4 g.cm⁻³ in order to minimise themass of the monocrystalline blades and, consequently, to limit thecentrifugal stress acting on the blades and on the turbine disc to whichthey are fixed;

[0021] a good tendency to monocrystalline solidification of turbineblades whose height can reach several tens of centimeters and the massseveral kilograms.

[0022] The superalloy according to the invention, which is suitable formonocrystalline solidification, has the following composition by weight:Co:  4.75 to 5.25% Cr:  11.5 to 12.5% Mo:  0.8 to 1.2% W:  3.75 to 4.25%Al:  3.75 to 4.25% Ti:    4 to 4.8 Ta:  1.75 to 2.25% C: 0.006 to 0.04%B: ≧0.01% Zr: ≧0.01% Hf: ≧1% Nb: ≧1% Ni and any impurities: complementto 100%.

[0023] The alloy according to the invention is an excellent compromisebetween resistance to creep and resistance to hot-corrosion. It issuitable for the manufacture of monocrystalline components, that is tosay, components which comprise a single metallurgical grain. Thisspecific structure is obtained, for example, by means of a conventionaldirectional solidification process at a thermal gradient, using ahelical or chicane-like device for selecting a grain, or a monocrystalnucleus.

[0024] The invention also relates to an industrial turbine blade whichis produced by monocrystalline solidification of the above superalloy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The features and advantages of the invention will be set forth ingreater detail in the description below with reference to the appendeddrawings.

[0026]FIGS. 1 and 2 are graphs illustrating the properties of differentsuperalloys.

DETAILED DESCRIPTION

[0027] An alloy according to the invention designated SCB444 has beenproduced with reference to the nominal composition listed in Table I. Inthis Table, the nominal concentrations of major elements of referencealloys IN939, IN738, IN792 and SC16 are also listed. TABLE IConcentrations by weight of major elements (%) Alloy Ni Co Cr Mo W Al TiTa Nb IN939 Base 19 22.5 — 2 1.9 3.7 1.4 1 IN738 Base 8.5 16 1.7 2.6 3.43.4 1.7 0.9 IN792 Base 9 12.4 1.9 3.8 3.1 4.5 3.9 — SC16 Base — 16 3 —3.5 3.5 3.5 — SCB444 Base 5 12 1 4 4 4.4 2 —

[0028] Chrome has an advantageous and dominant effect on the resistanceto hot-corrosion of nickel-based superalloys. Thus, tests have shownthat a concentration in the order of 12% by weight was necessary andsufficient in the alloy of the invention in order to obtain resistanceto hot-corrosion that is equivalent to that of reference alloy IN792under the conditions for hot-corrosion tests described below, whichconditions are representative of the environment created by combustiongases of some industrial turbines. A higher chrome content would notallow the volume fraction of the γ′ phase, which is necessary for goodcreep resistance of the alloy up to 1000° C., to be reached without thealloy becoming unstable in respect of the precipitation of fragileintermetallic phases which are rich in chrome in the γ matrix. Chromealso contributes to the hardening of the γ matrix in which this elementis preferentially distributed.

[0029] Molybdenum greatly hardens the γ matrix in which the element ispreferentially distributed. The quantity of molybdenum which can beintroduced to the alloy is limited, however, because the element has adisadvantageous effect on the resistance to hot-corrosion ofnickel-based superalloys. A concentration in the order of 1% by weightin the alloy of the invention is not detrimental to the corrosionresistance and contributes significantly to its hardening.

[0030] Cobalt also contributes to the hardening in the form of a solidsolution of the γ matrix. The concentration of cobalt has an effect onthe dissolution temperature of the γ′ hardening phase (γ′ solvustemperature). Thus, it is advantageous to increase the concentration ofcobalt in order to decrease the solvus temperature of the γ′ phase andto facilitate the homogenising of the alloy by means of heat treatmentwithout any risk of causing melting to start. Furthermore, it can alsobe advantageous to reduce the concentration of cobalt in order toincrease the solvus temperature of the γ′ phase and to benefit in thatmanner from greater stability of the γ′ phase at elevated temperature,which promotes resistance to creep. A concentration in the order of 5%by weight of cobalt in the alloy of the invention leads to an optimumcompromise between a good capacity for homogenising and good resistanceto creep.

[0031] Tungsten, whose concentration is in the order of 4% by weight inthe alloy of the invention, is distributed in a substantially equalmanner between the γ and γ′ phases and, in that manner, contributes tothe respective hardening processes thereof. Its concentration in thealloy is, however, limited because the element is heavy and has anegative effect on the resistance to hot-corrosion.

[0032] The concentration of aluminium is in the order of 4% by weight inthe alloy of the invention. The presence of the element causes theprecipitation of the γ′ hardening phase. Aluminium also promotesresistance to oxidation. The elements titanium and tantalum are added tothe alloy of the invention in order to reinforce the γ′ phase in whichthey are substituted for the element aluminium. The respectiveconcentrations of those two elements in the alloy of the invention arein the order of 4.4% by weight for titanium and 2% by weight fortantalum. Under the conditions for hot-corrosion tests described below,corresponding to the intended application, tests showed that thepresence of titanium was more favourable to the resistance tohot-corrosion than was the case with tantalum. However, theconcentration of titanium has been limited, on the one hand, by the factthat the element can have a negative effect on the resistance tooxidation and, on the other hand, because an excessively highconcentration of titanium can lead to a destabilisation of the γ′ phase.The total of the concentrations of tantalum, titanium and aluminiumroughly determines the volume fraction of the γ′ hardening phase. Theconcentrations of those three elements have been adjusted in order tooptimise the volume fraction of the γ′ phase, while keeping the γ′and γ′phases stable when maintained for long periods at elevated temperature,and taking into consideration the fact that the concentration of chromehas been fixed at approximately 12% by weight in order to achieve thedesired resistance to corrosion.

[0033] Alloy SCB444 has been produced in the form of monocrystals havingorientation <001>. The density of that alloy has been measured and foundto be equal to 8.22 g.cm⁻³.

[0034] After directional solidification, the alloy is substantiallyconstituted by two phases: the austenitic matrix γ, which is a solidnickel-based solution, and the γ′ phase, which is an intermetalliccompound whose basic formula is Ni₃Al and which precipitates mainlywithin the γ′ matrix in the form of fine particles measuring less than 1micrometer during cooling to the solid state. A small fraction of the γ′phase is also located within solid particles resulting from a liquideutectic transformation→γ+γ′ once solidification has ended. The volumefraction of the γ/γ′ eutectic phase is in the order of 1.4%.

[0035] Alloy SCB444 underwent homogenising heat treatment at atemperature of 1270° C. for 3 hours with cooling in air. Thistemperature is higher than the solvus temperature of the γ′ phase(dissolution temperature of the precipitates of the γ′ phase), which is1253° C., and less than the solidus temperature, which is 1285° C. Thetreatment is intended to dissolve all of the precipitates of the γ′phase, whose distribution of sizes is very wide in the coarse state ofdirectional solidification, to eliminate the solid γ/γ′ eutecticparticles and to reduce the chemical heterogeneities which areassociated with the dendritic solidification structure.

[0036] The interval between the γ′ solvus temperature of the alloySCB444 and its solidus temperature is very large, which allows readyapplication of the homogenising treatment without any risk of meltingand with the certainty of obtaining a homogeneous microstructure whichallows optimised resistance to creep.

[0037] The cooling which follows the homogenising treatment describedabove was carried out by hardening in air. In practice, the rate of thiscooling must be so high that the size of the particles precipitatedduring the cooling operation is less than 500 nm.

[0038] The homogenising heat treatment procedure which has just beendescribed is an example which allows the intended result to be achieved,that is to say, a homogeneous distribution of fine particles of the γ′phase whose size is no greater than 500 nm. This does not exclude thepossibility of obtaining a similar result by using a different treatmenttemperature provided that the temperature lies within the rangeseparating the γ′ solvus temperature and the solidus temperature.

[0039] Alloy SCB444 was tested after undergoing a homogenising treatmentas described above, then two annealing treatments which allow the sizeand the volume fraction of the precipitates of the γ′ phase to bestabilised. A first annealing treatment consisted in heating the alloyto 1100° C. for 4 hours with cooling in air, which leads tostabilisation of the size of the precipitates of the γ′ phase. A secondannealing treatment at 850° C. for 24 hours, followed by cooling in air,allows the volume fraction of the γ′ phase to be optimised. This volumefraction of the γ′ phase is estimated at 57% in alloy SCB444. Once allof the heat treatments are completed, the γ′ phase has been precipitatedin the form of cuboid particles whose size is between 200 and 500 nm.Cyclical hot-corrosion tests were carried out at 900° C. on alloy SCB444on an industrial corrosion bench with a burner. The cycle was asfollows: 1 hour at 900° C. in the corrosive atmosphere produced by theburner, then 15 minutes out of the oven at ambient temperature. Theburner operated with fuel loaded with 0.20% sulphur. A saline watersolution at 0.5 g.l⁻¹ NaCl was vaporized on the test piece at a rate of2.2 m³.h⁻¹. The test piece was coated every 100 hours with a deposit of0.5 mg.cm⁻² Na₂SO₄. For comparison, alloys IN738 and IN792 were testedat the same time. The criterion for corrosion resistance is the numberof cycles for which the first corrosion pits appear on the surface ofthe test piece.

[0040] The results of the corrosion tests are illustrated by the graphin FIG. 1. The start of corrosion at 900° C. occurs for cycle totalswhich are comparable for alloys SCB444 and IN792, which fulfils thestated objective.

[0041] Tests for creep under tensile stress were carried out on machinedtest pieces in monocrystalline bars of orientation <001>. The bars werehomogenised beforehand then annealed according to the proceduresdescribed above. Values for rupture times obtained at 750, 850 and 950°C. for different levels of stress applied are listed in Table II. TableII Service lives with creep of alloy SCB444 Temperature (° C.) Stress(MPa) Rupture time (h) 750 725 134 750 650 612 750 600 1152 850 500 43.1850 425 168.5 850 300 3545/>3456 950 250 115/135 950 200 551/544 950 180578 950 140 2109 950 120 3872

[0042] The graph in FIG. 2 allows a comparison of the rupture times withcreep obtained for alloys SCB444, IN738, IN792 and SC16. The stressapplied is plotted on the abscissa. The value of the Larson-Millerparameter is marked on the ordinate. This parameter is given by theformula P=T(20+log t)×10⁻³, where T is the creep temperature in Kelvinand t is the rupture time in hours. This graph shows that the creepresistance of alloy SCB444 is distinctly superior to that of alloyIN792.

[0043] The inspection of the microstructure of the test pieces of alloySCB444 at the end of the creep tests demonstrated the absence ofprecipitation of fragile intermetallic particles which are rich inchrome and which are capable of appearing when maintained for sustainedperiods at elevated temperature in nickel-based superalloys where the ãmatrix is over-saturated with additive elements.

[0044] Manufacturing tests on monocrystalline components of super-alloySCB444 demonstrated that it was possible to cast a large range ofcomponents whose mass can range from a few grammes to more than 10 kg,with various levels of complexity. The growth of components according tothe crystallographic orientation <001>is promoted and dominant and thepresence of grains that are orientated in a random manner is minimised.The liquid metal is stable in the sense that it does not react with thematerials commonly used in the manufacture of moulds. The phenomenon ofrecrystallisation which can occur during homogenising treatment atelevated temperature is absent in the case of alloy SCB444.

We claim:
 1. A nickel-based superalloy, suitable for monocrystallinesolidification, characterised in that its composition by weight is asfollows: Co:  4.75 to 5.25% Cr:  11.5 to 12.5% Mo:  0.8 to 1.2% W:  3.75to 4.25% Al:  3.75 to 4.25% Ti:    4 to 4.8% Ta:  1.75 to 2.25% C: 0.006to 0.04% B: ≦0.01% Zr: ≦0.01% Hf: ≦1% Nb: ≦1% Ni and any impurities:complement to 100%.


2. Industrial turbine blade produced by monocrystalline solidificationof a superalloy according to claim 1.